1. Retirement! We've had three models--profs who really retire and no longer come in, profs forced out (he finally ran out of money after his last grant was not renewed) and profs who are tapering off to retirement but take Emeritus status even with a grant or two to make way for fresh blood (we have 2 or 3 of these). These voluntary Emeritus profs are really considering the interests of the department in going Emeritus, and as a bonus often end up as wonderful mentors (if you care enough to make room for someone, you also care enough that your replacement does well).
2. Old prof dies at the bench (yes, this happened--he had a heart attack in the lab at age 85 and died a week or so later in the hospital).
3. Non-deadwood, non-research active. These are the profs our Chair has been successful at getting the Dean to open lines to "replace". They are still very active in service to the department and University, and take much higher than normal teaching loads on service classes to free up the more active folks to teach the majors/grad students and have lighter loads. These people are really helpful in accommodating sabbaticals and family leaves without crushing the TT profs with work, so I really appreciate them!
4. Expansion. Prodigal U is growing both the number of undergrads and grad students, and as a service department, we got some extra TT lines in the past.
That said, I think our run of hiring is about to end. Given how Wall Street ate most people's retirement money, I am thinking the wave of retirements is likely to end for a while. Let's face it, unless you do research that requires arduous field work, most STEM professorial jobs are not particularly strenuous. It is easy to hang on for a long, long time, especially if your retirement savings have evaporated.
#2 is not only a random occurrence, but it seems really unlikely to happen again soon!
We've been really lucky to have a Dean who is interested in increasing the research profile at Prodigal U, so we've gotten to add some TT faculty to replace not just retirements, but also some of the research inactive faculty in the department. Now that Prodigal U is in budget crunch mode, we're not getting any more of these. And any expansion workload issues will similarly have to be eaten by the department.
I fear for those on the job market this year.